The integrated Abelson murine leukemia virus (A-MuLV) genome cloned in bacteriophage vector was used to analyze the functional nature of the different segments of the viral genome. Construction of mutants having sequential deletions of the A-MuLV-specific abl gene revealed that only the proximal 40% of abl with its associated 5' helper viral sequences is required for fibroblast transformation. To define regions of long terminal repeat (LTR) required for A-MuLV transforming gene function, deletion mutants encompassing different domains were constructed and assayed following transfection of NIH/3T3 cells. Deletions involving only TATA and CAAT sequences showed reduced transforming activity, whereas deletions extending to the end of the LTR abolished transforming activity. Removal of both 72-bp repeats completely abolished the transforming activity. Interestingly, ligation of wild-type 3' LTR to the defective 5' LTR subgenomic clones restored the transforming activity. Nucleotide sequence analysis of the complete proviral genome of A-MuLV had been determined. The amino acid sequence of the abl gene, derived from the primary DNA sequence, has significant homologies to other tyrosine-specific protein kinases encoded by src, fes, fps and yes onc genes.